The present invention relates to a technique of enhancing the evidence of digital signature.
There is a technique for enhancing the evidence of digital signature (hereafter, called signature) in which when a signature is made, the signature log information up to that time is reflected, and the information about the generated signature is added to the signature log as a new signature log entry (for example, see patent documents JP-A-2001-331104 or JP-A-2001-331105). This method makes the generated signature have a chain structure. When the evidence is validated, chain verification is made in addition to signature verification, thus making the interpolation difficult.
In this method, the signatures with a continuous chain are decided allowable, but in order to definitely prove the correctness, the signature log is desired to include a reliable, proper signature log entry. In order to generate this reliable signature log entry in the above method, periodic publication of signature log entries is proposed as one of the countermeasures.
Each user who makes his or her signature by using the above method publishes the latest signature log entry at fixed intervals or at every constant times. If the signature log entry is published on an issue such as newspaper or gazette (hereafter, called the newspaper), the legal signatory can designate the unfair signature log entries produced by the other persons, so that it can be brought to light. Thus, the disclosed signature log entry can be treated as a proper signature log entry. In addition, since the disclosed signature log entry is made available to the public, it is difficult to cancel afterward or alter dishonestly.
In the above technique, when the number of users is large, it is realistically difficult for all users to publish their signature log entries in the newspaper. Therefore, a more practical scheme is desired.